I got an LED flash light at Walmart that is like 1/2 inch by 2 inches and about 1/4 thick. It is surprisingly bright and does not run
down easily. LED's (light emiting diodes) use hardly any electricity
I use it as a back up and it got me to the truck when my head
lamp's batteries died.
This thing is really small. It requires that you hold down the
button but I'm sure there are others that fill the bill.
http://www.theledstore.com/prtecledfl.html